1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding a sheet, and an image forming apparatus having the sheet feeding apparatus. And particularly, the present invention relates to drive control for a sheet feeding motor that is mounted in the sheet feeding apparatus.
2. Description of the Related Art
In an image forming apparatus of a high-speed electrophotography system with many printouts per unit time, it is necessary to control a sheet conveyance operation accurately and to maintain stability of high-speed sheet conveyance. In order to achieve such a high-speed sheet conveyance, a hybrid type stepping motor of high torque and high speed is used conventionally.
A cheap and efficient PM type stepping motor is now being developed. For example, Japanese Laid-Open Patent Publication (Kokai) No. 2002-366001 (JP 2002-366001A) discloses a method for controlling to drive a cheap PM type stepping motor without causing step-out. In this method, an initial operation, which rotates a motor by a phase signal of at least one or more patterns of excitation patterns repeated at a frequency within a self-starting range, is executed when starting the motor at a beginning after excitation current for the motor has been severed after power-on of a printer due to a certain event. For example, when starting a sheet conveyance operation, the phase of the motor is synchronized with the phase of the phase signal, and the printer is shifted to a paper feeding operation immediately after the above-mentioned initial operation.
However, a cheap PM type stepping motor cannot withstand a rapid torque variation. Therefore, when one object is driven by a normal rotation of the motor and another object is driven by a reverse rotation of the motor, the motor tends to cause step-out due to a backlash of a mechanical gear, etc.
In order to prevent the step-out, although it is effective to increase the number of the excitation patterns repeated at the frequency within the self-starting range, the increase of the number of the excitation patterns extends standby time, which causes a problem to decrease productivity of a print process.